1. Field of the Invention
This invention relates to an aid for use in, and a process for, the machining of metals, and more particularly, to an aid which can be applied to cutting tools, drills, cut-off wheels, grinding wheels and coated abrasive products to accelerate the machining process.
2. Description of Prior Art
Abrasive products and cutting tools employed to remove metal stock generally fail, i.e., they lose cutting effectiveness, after varying periods of use, especially when they are employed to modify high temperature metal alloys. In the majority of cases involving high temperature alloys, one of the most prominent causes of failure resides in the fact that the freshly exposed or cut alloy surface is highly reactive and this "nascent" area is subject to the formation of a weld juncture which exerts an extremely high shear force against the abradant or cutting material. It is also quite clear that the welding becomes far more acute under conditions of high temperature. Although this inherent problem is encountered in all forms of grinding and cutting, it is particularly troublesome in the case of coated abrasives where the surface is not renewed as in grinding wheels and such. Since coated abrasives essentially rely on only a single layer of abrading particles, it has been found, to date that little can be done to improve their efficiency beyond a specific point.
It is also well-known that cutting tools, grinding wheels and coated abrasives become capped with a metal swarf, i.e., loaded. In addition to the problems of welding and loading, there exists what is known as "glazing", wherein the cutting edges become deformed by the extremely high temperatures to which the cutting points and edges of cutting tools, grinding wheels, abrasives and the like are exposed in grinding and cutting, causing plastic deformation of the cutting points and edges.
It appears that there is a direct inter-relationship between the foregoing factors and temperature. Factors which normally tend to elevate the temperature at the work surface (interface) also promote welding, chemical reactions, glazing generation of internal workpiece stresses, as well as burning of the workpiece surface, which adversely affect the metallurgical structure at the surface. These factors are present in all cutting techniques but they are substantially more severe in the super alloys due to their high temperature and low-thermal conductivity characteristics.
Attempts to externally improve grinding and cutting ability have included the application of grease sticks, oils and other lubricants to the workpiece surface during the grinding and/or cutting operation. Also, attempts have been made by various manufacturers to incorporate aids into abrasive belts and grinding wheels in a permanent fixed manner during fabrication.
These aids include solids, liquids and gases which serve generally to improve conditions within the restricted cutting or grinding area. Another common approach has been to incorporate within the metal to be machined quantities of sulfur, selenium and/or lead to provide improved machinability. A similar result can be attained by the use of grinding aids containing sulfur, halogens (e.g., fluorine, chlorine) and phosphorus. The most commonly used grinding aids are in the form of liquids and include water, soluble oils, mineral and fatty straight cutting oils, as well as those that are sulfurized and chlorinated. The latter, as stated above, may be effective for certain metals but are not entirely useful or desirable for certain super alloys and titanium due to chemical reactions between these chemicals and the metal surface being machined or ground. Greases and hard waxes are not effective except in reducing the loading of relatively soft metals such as aluminum, brass, etc. Other lubricants such as chlorinated and fluorinated hydrocarbons have been used to reduce heat generation in the area of the workpiece and grinding tool interface.
As a class, the presently employed aids are more or less toxic and their use and the surrounding environment must be strictly controlled so as to minimize any danger to the health of the operator. In the case of lead, bismuth, sulfur, mercury or halogen containing aids, gases generated during use can affect the workpiece and/or be toxic and care must be exercised in prolonged use with continual inspection and testing. In this regard, it should be observed that various specifications by the government and major aerospace manufacturers preclude the use of certain halogen materials in proximate relation with the workpiece as well as the operator.
The described aids have been used on standard materials with varying degrees of success but have been limited in the field of space-age super alloys to safeguard the surface integrity of the workpiece. Further, the enactment and enforcement of laws protecting the health of factory workers now requires warning labels when certain of these aids are included for example as a supersize coat on coated abrasives.
I have discovered that the problems mentioned with respect to the prior art grinding and cutting aids can be overcome and that any grinding or cutting process on any metal or other workpiece can be accelerated while also prolonging the useful life of the tool performing sad process by bringing the workpiece into relatively moving contact with a grinding or cutting edge in the presence of an effective amount of a grinding or cutting aid comprising at least 10% by weight of a solid compound free of sulfur and/or halogen and having a melting point in the range of 70.degree. F. to 1000.degree. F., a decomposition temperature at least 100.degree. F. above the melting temperature and a latent heat of melting greater than 10 cal/gm. A typical compound having the above characteristics is sodium nitrite. Generally, inorganic compounds are preferred because of their lower cost of manufacture.
U.S. Pat. No. 3,595,634, issued to Sato discloses the employment of 3 to 10% by weight of sodium nitrite as one of the initial ingredients of his formulation and process for making grindstones. Sato teaches the use of a highly effective and superior anticorrosive chemical compound, namely, amine nitrite, which, Sato teaches, is the reaction product of amines (120 to 250% of the equivalent weight of the epoxide) with the 3-10% of sodium nitrite in presence of heat and pressure when mixed with epoxy. According to Sato, there is no sodium nitrite in the final product produced by his process.
U.S. Pat. No. 2,529,722, issued to Chester relates to a buffing and polishing composition for soft base metals which uses iron tailings as abrasive elements with alkali metals in the form of salts or complex oxides. To the foregoing, Chester adds a minute quantity of an electrolyte. Sodium nitrite is mentioned among other suitable materials as an electrolyte and only in minute proportions, namely, 1/16 to 1/4% by weight as a rust inhibitor to prevent oxidation of the iron tailings in water. This amount would be insignificantly inadequate to perform the heat absorption function required of the aid of this invention.
U.S. Pat. No. 3,607,161, issued to Monick discloses a scouring composition which comprises a cationic surface-active compound and a water soluble abrasive. Monick lists in excess of 60 water-soluble salts which act as abrasives, one of which is sodium nitrite. Sodium nitrite in crystalline form is equated to an abrasive, and not taught to be an aid for some other abradant in lowering the grinding temperatures.